Proposed is a prospective dynamic cohort study to determine the prevalence and incidence of HIV infection in recently initiated young injection drug users (IDUs), or "initiates", and young non-injection drug users or NIUs (e.g., smoke crack, sniff heroin, etc.), and to identify correlates and risk factors for injection and for transition into IDU. Eligibility or IDUs is age 15-30 years with initiation within past 5 years; NIUs is age 15-30 with use started >1 but <10 years before entry. The 400 initiates and 200 NIUs will be recruited using street outreach techniques to a stationary or mobile clinic and East and Central Harlem. Recruited over 5 years, participants will undergo interviews and testing for HIV, HBV and HCV at entry, 6 and 12 months post-entry. Building upon our prior work, the interview will include "new" variables )physical/sexual abuse, psychiatric condition, social networks/social support, type/frequency non-injection drugs); established variables will include demographics, drug and sex practices, and circumstances of IDU initiation. Analyses for correlates of infection will include logistic regression, and analysis of incidence will include person-time and poisson regression. Analysis of transition into IDU will include a case-control analysis of initiates and NIUs matched by age and interview interval for the year prior to initiation of the IDU case, using conditional logistic regression, prospective transition will use survival-methods. This study will provide new information to develop strategies and to better target primary prevention for blood-borne infections and injection drug use.